This invention relates generally to papermaking, and more particularly to equipment employed with papermaking machines.
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper xe2x80x9cstockxe2x80x9d) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a xe2x80x9cforming fabricxe2x80x9d, provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i.e., the xe2x80x9cmachine sidexe2x80x9d) of the upper run of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a xe2x80x9cpress felt.xe2x80x9d Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a xe2x80x9cbattxe2x80x9d layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
The paper stock is fed onto the forming fabric from a device known as the xe2x80x9cheadboxxe2x80x9d, which applies a jet of stock onto the forming fabric. A xe2x80x9cbreast rollxe2x80x9d is located beneath the headbox and serves as the upstreammost roll over which the forming fabric is conveyed. In many paper machines, and particularly more modem machines, a xe2x80x9cforming boardxe2x80x9d is located just downstream of the breast roll, typically in the area beneath the portion of the forming fabric that receives the jet of paper stock. In this location, the forming board can support the forming fabric against deflection due to the force of the jet, and can provide well-defined drainage for the paper stock.
A typical forming board includes a series of blades (usually formed of ceramic or, more recently, polyethylene) that extend substantially parallel to one another across the width of the fabric and that are separated by gaps that extend in the cross-machine direction. The degree of open area provided by the gaps can impact the amount of drainage occurring at the forming board. Many forming boards also include a lead blade with a wedge-shaped xe2x80x9cnosexe2x80x9d on its leading edge that serves to xe2x80x9cdoctorxe2x80x9d water beneath the lead blade.
Because the configuration of the forming board can impact drainage, which, in turn, can impact paper quality, the sizes of the blades and the spacing therebetween should be considered carefully during design and installation. In fact, in many paper mills, the blade positions are adjusted for each different type of paper made on the machine. Also, often the paper mill will match the forming board blade size and spacing to match that of other foil units that are positioned downstream of the forming board, and it is typically desirable to position the blades such that the gaps between blades are of uniform width. With some forming boards, the degree of open area is altered by installing blades of different widths (which can be somewhat laborious, particularly if numerous adjustments are required to attain acceptable paper machine performance). For other forming boards, spacing between blades can be adjusted manually, with each blade being repositioned and fixed into place. However, this type of adjustment can not only be time-consuming, but also may result in the spacing between blades being non-uniform. Thus, it would be desirable to provide a forming board having a configuration that would enable the open area to be adjusted without the installation of replacement blades and that would provide substantially uniform spacing between the blades automatically.
The present invention is directed to a forming board for a papermaking machine. In a first embodiment, the forming board comprises: a support; a transversely-extending lead blade attached to the support, the lead blade having an upper surface; a plurality of transversely-extending trailing blades, each of the trailing blades having an upper surface, a leading edge and a trailing edge; a mounting unit for each of the plurality of trailing blades, the mounting unit being attached to a respective trailing blade and to the support such that the upper surfaces of the lead blade and the trailing blades are substantially coplanar and such that gaps are defined between the trailing edges and leading edges of adjacent blades, the gaps being of substantially uniform width; and a drive unit attached to the mounting unit and to the support, the drive unit being configured to drive the trailing blades simultaneously to different longitudinal positions relative to the support, wherein the widths of the gaps vary but remain substantially uniform for each different longitudinal position. In this configuration, the gaps between the blades of the forming board can be maintained at substantially uniform width as the positions of the blades are adjusted for different paper grades.
In certain embodiments, the drive unit comprises a longitudinally extending positioning shaft, the positioning shaft being rotatably mounted to the support, and each trailing blade is mounted to the support via a mounting unit that engages the positioning shaft. In some of such embodiments, the positioning shaft includes a plurality of threaded sections, each of the threaded sections having a different thread pitch, and each mounting unit includes a threaded bore that is complimentary to one of the threaded sections of the positioning shaft.
As a second aspect, the present invention is directed to a forming board for a papermaking machine comprising papermaking machine, comprising: a support; a transversely-extending lead blade fixed to the support, the lead blade having an upper surface; a plurality of transversely-extending trailing blades, each of the trailing blades having an upper surface, a leading edge and a trailing edge, the blades being attached to the support such that the upper surfaces of the lead blade and the trailing blades are substantially coplanar and such that gaps are defined between the trailing edges and leading edges of adjacent blades; and a drive unit attached to the support and with the trailing blades, the drive unit being configured to drive the trailing blades simultaneously between a first position, in which the each of the gaps has a first width, the first widths of each of the gaps being substantially uniform, and a second position, in which each of the gaps has a second width that is different from the first width, the second widths of the gaps being substantially uniform.